1. Field of the Invention:
This invention relates to a PG yoke position detecting apparatus for detecting a rotation reference position of a yoke, referred to as a PG yoke, used in detecting the rotation reference position of a recording medium in a floppy disc system for recording and/or reading the picture or data of an electronic still camera or video floppy disc (VFD).
2. Description of the Prior Art:
In a floppy disc for an electronic still camera the general standards whereof have been roughly defined, the disc includes a center hub provided with a metal plate for being attracted by a magnet. Upstanding from a portion of the metal plate is a PG yoke used for detecting a picture or data recording start position. FIGS. 1(a) and 1(b) illustrate a conventional PG yoke position detecting mechanism, in which FIG. 1(a) is a front view and FIG. 1(b) a sectional view taken along line A--A of FIG. 1(a). A magnetic recording medium 2 accommodated in a hard case or cassette 1 includes a center hub having a yoke 5 from a porition of which an upstanding PG yoke 6 is formed. The electronic still camera that receives the cassette 1 includes a motor spindle 3 and an attracting magnet 4. The magnet 4 attracts the yoke 5 of the recording medium 2, thereby holding the recording medium 2 fast to the motor spindle 3 so that medium 2 may be rotated by the spindle 3. The magnet 4 produces a flux which exits from the end face 6.sub.1 of the PG yoke 6. When the spindle motor (not shown) is rotated, thereby rotating the recording medium 2 and, hence, the PG yoke 6, this causes an induced electromotive force to be produced in a PG coil 7, which is arranged directly opposite the PG yoke 6, at a rate of once per revolution of the recording medium 2. However, since the electromotive force detected by the PG coil 7 varies in dependence upon the rotating speed of the recording medium 2, it is difficult for the PG coil 7 to detect a reference position in a stable manner. Moreover, since the polarity of the induced electromotive force will differ depending upon the pole orientation, either N or S, of the attracting pole of magnet 4 provided on the electronic still camera, inconveniences are encountered in terms of manufacture control, adjustment and interchangeability of the apparatus. Though using a Hall device will solve the problem of a variation in amplitude with the rotational speed of the recording medium, the problem of a difference in polarity stemming from the magnetic pole orientation of the magnet 4 remains. Futhermore, if a Hall device is used, the extracted signal is small in magnitude. As a result, highly reliable position detection is difficult with such an apparatus in an environment in which field noise is prominent. Further, with regard to the detection of field leakage from the PG yoke in the conventional PG yoke position detecting apparatus, which flux component should be picked up as a signal to contribute to an improved S/N ratio is not given any consideration whatsoever. This fact also makes it difficult to realize an apparatus capable of highly accurate, reliable detection.